I have been doing a little research on the JAP engine subject, i have been given an old copy of Old Bike Mart from May 2005 with an article on Cameron Engineering which says "the castings come from MFC Patterns & Castings of Doncaster".
I followed this up with another phone call to the guys at MFC who confirmed that this was indeed the case and that they have been making parts for these engines since the eighties, also saying that the parts are available to everybody.
I had another very interesting conversation, As Mike Cullingworth of MFC said "it has been said on more than one occasion that we are our customers best kept secret", they have produced parts for lots of our leading motorcycle companys including parts for Matchless G50,G45,G85,G80,AJS,BSA Golstar,Ariel HT5,Vincent
HRD,Manx,Inter,Commando Nortons,Triumph,Honda 6, plus other Japanese and italian race bikes!
Not bad for a small UK company! I have added a link to the MFC website, Cheers John
http://www.mfcpatterns-castings.co.uk